Pop star Justin Bieber has made it clear that Selena Gomez is his "Favorite Girl." On Sunday, the teen heartthrob gave his girlfriend a late birthday surprise -- the big day was actually on Friday, July 22 -- during her concert in Costa Mesa, California, as he took the stage himself and sang Justin Timberlake's "Cry Me a River." The song's about a breakup, but maybe the tune has a special meaning for the couple? Despite the lyrics, Selena, who turned 19, was obviously shocked -- in a good way!

The appearance was only part of Selena's special weekend. Two days earlier, she and Justin were spotted at Disneyland in Anaheim, California, during the day and dining out at Geisha House in Hollywood at night.

In case you had any remaining doubts that he's the perfect boyfriend, JBiebs even sent some sweet messages about Selena via Twitter. "Happy Birthday @SelenaGomez gonna be a special day," he tweeted on Friday. He topped it all off on Sunday after the show: "@selenagomez u did great tonight. proud of u." Aww! I know young love rarely lasts, but I can't help but root for these two. What about you?